Changes in blood pressure and heart rate during fixed-interval responding in squirrel monkeys

Episodic and sustained increases in heart rate and mean arterial blood pressure can occur with recurring patterns of schedule-controlled behavior. Most previous studies were conducted under fixed-ratio schedules, which maintained a consistent high rate of responding that alternated with periods of no responding during times when the schedule was not in operation. The present study examined changes in heart rate and blood pressure under fixed-interval schedules which maintained a range of rates that varied from little or no responding at the beginning of the fixed interval to high rates at the end of the interval. The relations of cardiovascular function to rate of responding were examined. Squirrel monkeys prepared with arterial catheters were trained to respond under fixed-interval schedules of electric-shock presentation. The duration of the interval was varied across sessions and cardiovascular parameters were examined. Local rates of responding were typically near zero during timeout periods, low at the beginning of each fixed-interval cycle, and then increased as the fixed interval progressed. At most schedule durations, arterial blood pressure and heart rate levels were lowest at the beginning of the interval cycles, increased as the rate of responding increased, and then decreased during the timeout periods. At all parameters studied, there was a direct relationship between changes in response rate within fixed-interval cycles and changes in heart rate and blood pressure. The results suggest that a much closer concordance of these cardiovascular parameters and schedule-controlled responding is obtained by examining ongoing behavior as it occurs within the contingencies by which it is maintained.